Description: The lack of a robust set of markers that differ between mature CD4+CD8- thymocytes and most antigenically naive peripheral T cells, that can identify recent thymic emigrants in humans to assess thymocytopoiesis, has made it difficult to test the hypothesis that recent thyrnic emigrants would be expected to comprise a treater percentage of the peripheral T cell compartment in the neonate compared to the adult, since thymocytopoiesis is likely to occur at a high ate prior to puberty, particularly during early development. Such markers also might allow a better assessment of the thymocyte function in clinically relevant situations, as during treatment of HIV infection or following bone marrow transplantation. This project seeks to identify multiple genes that are differentially expressed by CD4+CD8&Auml; thymocytes versus adult antigenically naive CD45RAhigh CD4 T cells, either directly after isolation, or after stimulation in vitro for six hours. These studies will use a specialized gene microarray assay for approximately 11,000 named genes or anonymous expressed sequence tags (ESTs) expressed by human lymphoid cells. It will be determined whether neonatal CD4 cells express these genes in a thymocyte-like pattern, consistent with their likely enrichment in recent thymic emigrants, rather than an adult CD4 T cell pattern. The final aim is to generate a human fetal thymus cDNA library for high throughput random sequencing and the identification of novel ESTs. If such novel ESTs are identified, they will be included in the microarray analysis, and may allow the identification of additional genes that are differentially expressed by particular T-lineage populations.